So I got this cannon lying around here basically 80% done and it can also fire with the necessary preparations, but it's still far away from something you can pick up, pull the trigger and shoot.

It uses a large piston valve with the hammer valve of an old fire extinguisher to vent the pilot. I can shoot it by hitting that valve with a large hammer. But i'm completely lost on an actual trigger mechanism as that would require a very tough spring.

Don't think a toy gun's hammer would do much tho, i even doubt if the hammer mech from an actual gun would do anything as the principle of that is basically the large heavy hammer transferring all it's energy to the firing pin's tiny surface, and the return spring of a firing pin is also pretty weak. I can barely push the valve down with my thumb even without any pressure on the pilot.

I think you'd need to cut a sten gun's barrel off and mount it so that the bolt of the gun would hit the valve when you pull the trigger

The staple guns seem plausible, but i think i would need a very decent one to even attempt opening the valve. Were talking a push button type valve from a CO2 extinguisher that normally uses a hinged lever to open it, and the pilot pressure is roughly around 500/650 psi. Last time it shot a ball of kitchen paper through one side of an old fake leather shoe at point blank range.

I remember reading something on this forum about some kind of free floating valve? I probably said it totally wrong but i think they use them on PCP rifles, had something to do with the valve's stem ending in a separate chamber that was equally pressurized as the main chamber so you only needed a weak spring to keep it in place and a lot less force to open it? Hope that made some sense :p

Yes that's the one i think! Bought some wire rod, washers and O-rings yesterday, gonna try and cobble something together today. If it's really that easy to push open i could probably use some kind of fast acting servo to open it, or maybe even a very small pneumatic piston leaching air from a mini air chamber operated by a blowgun...

Thanks! Is that the same type of valve the marshmallow cannon uses? :p